Magnesium alloy



Patented Oct. 2, 1951 MAGNESIUM ALLOY Zllhomas Iieontis, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich., a. corporation of Delaware N Drawing, Application Octqbgr 3, 1949, Serial No. 119,396

The invention relatesto an improved magnesium-base alloy exhibiting hi'gli creep resist ance and tensile strength at elevated-temperatures.

V In an application for Letters Patent, Serial No. 119,395, filed con-currently herewith by me, now abandoned, there is 'setforth a-n improved m nesium-base. alloy exhibiting high creep resistance and tensile strength at elevated temperatures which is based upon alloying neodymium and praseodymium with magnesium. Limited amounts of lanthanum normally associated with neodymium and praseodymiu-m also may be present, although superioralloysare obtained without. the lanthanum addition.

In further researches with. the foregoing .alloys, I have discovered that still greater, im- Drovement. in creep resistanceandelevated temperature tensile properties results when cerium is also alloyed with the magnesium along with the neodymium and praseod-ymium.

;Accordingly, the alloy of the present. invention consists of a base of magnesium with which is alloyed the three elements of the ceria earth group: cerium, neodymium, and praseodymium, their total amount being between about 1 and per cent by weight of the alloy, the balance being magnesium and incidental impurities, if any. It has been found that the usual small amounts of other ceria metals normally associated with above-specified ceria metals may be present in limited amount in the alloy without significant detrimental effects. Except for lanthanum, which may amount to as much as 10 per cent of the ceria earth metal content of the alloy, the other minor ceria earth metal impurities are generally less than about 3 per cent of the specified ceria earth metal content of the alloy. These minor ceria earth metals include Sm, Tb, Y. Insofar as known, these minor practically unavoidable alloying constituents have no measurable elfect on the tensile properties of creep resistance of the alloy.

One source of the metals neodymium and praseodymium for use in the alloy is commercial didymium which consists largely of these metals together with inconsequential amounts of some other ceria earth metals including sarmarium, terbium, yttrium and lanthanum which may be present in amount up to about 10 per cent of the ceria earth metal content of the didymium. The following is an example of a commercial didymium: Ce 0.8%, Nd 72.3%, Pr 7.9%, La 8.8% (by difference from the total ceria group rare earth metals content of 89.9%), Fe 7.05%, Si

2 Claims. (01. 75-168) 1'. f 0.56%, Cr 0.75%; other impurities 2.8% including small amounts of sarmarium, terbium, yttrium (by difference from 100%). The. other ceria earth metal for use in the. alloy, viz., cerium, normally contains small amounts of other ceria earth metals including neodymium, praseodymium and lanthanum in amount upto about 5 per cent of, the weight of the cerium. The following is an analysis of a commercial cerium: Ce 92.2%, Nd 0.9%, Pr 0.3%, La 1.4% (by difference from. thetotal rare earth; metals content; of 9%.81959 Fe 237%, Si. 0.0.2 ,Cr' 0.5 other. impurities 1.4% (by difference irom..100%). In. preparing the alloy, the magnesium, .:is

preferably-first melted; and then the specified ceria earth .metalsiare-added... The meltingrof the magnesium is 'most suitably. accomplished in anopentop-crucibleusing: a thickening ..magnesi-um foundry flux for protection from..-the; atmospheres" The usual thickening magnesium foundry flux may be used such-as one composed of-'-20% Kel; 50% -MgCl2, 15% CaFz and MgO in amount between about 1 and 3 per cent of the weight of the melt, although other amounts may be used. The flux has the property of being fluid at the commencement of melting and becoming crust-like after prolonged heating so that a crust-like cover of dried flux forms over the melt after a time. Before the crust forms, the specified ceria earth metals are introduced into the molten magnesium, preferably by holding them in a ladle and moving the ladle about in the melt so as to wash out the rare earth metals from the ladle and disperse them through the melt. After alloying the specified rare earth metals with the melt, it is desirable to cast the alloy promptly to avoid loss of the added metal to the flux which, if it contains magnesium chloride, is slowly decomposed by these rare earth metals.

In the event that the alloy once made is to be remelted, a process in which the flux is ordinarily in contact with the metal for a relatively long time compared to the alloying time, it is desirable, if not necessary, to employ a saline flux, free from MgClz, which remains fluid while molten, such as composed of 57% KCl, 28% CaClz, 12.5% BaClz, and 2.5% of CaFz in place of the crust-forming or thickening type of flux.

The following analysis is of an alloy made in accordance with the invention using the commercial didymium and cerium noted above: Ce 0.91%, Nd 0.54%, Pr 0.12%, La 0.12%, total rare earth 1.69%, balance magnesium of 99.7% purity.

The tensile properties are set forth, for the temperatures indicated, in Table I.

Table I Temmnb 115123;? Load in 1,000 s, p. s. 1.

3? tion in 2 Inches TY S TS Room 3. 7 13. 5 27. l 400 12.5 13.4 20.1 v 600 35. 5 10. 12.

TYS=Tensile yield strength, the stress at which the stress strain curve deviates 0.2% from the modulus line.

TS==Tensile strength, the stress at which the ratlo of stress to strain is a maximum.

The creep limits for 100 hours load application for the same alloy composition are set forth in Table II.

Table II Load in 1,000's p. s. i.

Tempgra- 0.1% Creep 0.2% Total 0.5% Total Extension Extension Extension of cerium, neodymium and praseodymium. A preferred proportion of cerium to the other two rare earth metals is an amount equal to the sum of the amounts of the other two.

It will be noted that the usual commercial forms of cerium and didymium contain minor amounts of other metals as impurities, such as iron, silicon and chromium. These impurities do not cause any harm because they are either insoluble in the magnesium and are, therefore, precipitatedand settle out of the melt during alloying, or are present in such small amount as to produce no significant effects in the final alloy.

I claim:

1. A magnesium-base alloy containing the rare earth metals cerium, neodymium and praseodymium. the balance being magnesium, the amount of said rare earth metals totaling between about 1 and 10 per cent of the weight of the alloy, the proportion of cerium being 25 to per cent of the total amount of rare earth metal, and the amount of neodymium being at least four times that of the praseodymium.

2. A magnesium-base alloy containing about 0.9 per cent of cerium, 0.54 per cent of neodymium, 0.12 per cent of praseodymium, and 0.12 per cent of lanthanum, the balance being magnesium.

THOMAS E. LEONTIS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,121,292 I-Iaughton et a1. June 21, 1938 2,221,319 Altwicker et al Nov. 12, 1940 2,464,918 Ball et a1. Mar. 22, 1949 FOREIGN PATENTS Number Country Date $9,586 Great Britain Feb. 5, 1931 

1. A MAGNESIUM-BASE ALLOY CONTAINING THE RARE EARTH METALS CERIUM, NEODYMIUM AND PRASEODYMIUM, THE BALANCE BEING MAGENSIUM, THE AMOUNT OF SAID RARE EARTH METALS TOTALING BETWEEN ABOUT 1 AND 10 PER CENT OF THE WEIGHT OF THE ALLOY, THE PROPORTION OF CERIUM BEING 25 TO 75 PER CENT OF THE TOTAL AMOUNT OF RARE EARTH METAL, AND THE AMOUNT OF NEODYMIUM BEING AT LEAST FOUR TIMES THAT THE PRASEODYMIUM. 